1. Field of the Invention
The present invention relates to the field of laser processing, and, in particular to a system in which a laser machining apparatus automatically performs focus position detection as a required preparatory operation to perform laser machining, stores detected data in a memory, and automatically performs focus position determination.
2. Description of the Background Art
A two-dimensional laser machining apparatus has been employed to cut a plate workpiece in contactless relation at a high speed and with a high accuracy. The two-dimensional laser machining apparatus controls a machining head along X, Y and Z axes, and cuts a workpiece using a laser beam which is introduced from a laser oscillator and outputted from the tip of the machining head. Usually, from a machining quality point of view, it is desirable that the laser beam be focused on the surface of the workpiece, so that when the workpiece is cut using the laser beam, it is necessary to perform a focus setting operation distinct from the actual machining operation. During actual machining, based on the height of the machining head determined by the focus setting operation, a height detecting sensor is mounted to the tip of the machining head, and machining is performed by following the outline of the workpiece so as to keep constant the distance from the tip to the workpiece by using the detecting sensor.
With reference to FIGS. 8 and 9, the general configuration of a two-dimensional laser machining apparatus will be explained hereinafter. Numeral 1 indicates a machining head which is mounted for movement along a Z axis 10. Numeral 2 indicates a distance sensor which functions as a focus detection means and is mounted on the tip of the machining head 1. Numeral 11 indicates a Z axis guide which guides the movement of the machining head 1 driven by the motor M3 in the direction of arrow Z; numeral 12 a Y axis guide which guides the movement of the machining head 1 driven by the motor M2 in the direction of arrow Y; and numeral 13 an X axis guide which guides the movement of the machining head 1 driven by the motor M1 in the direction of arrow X. The motors M1 through M3 are driven by a drive signal M from an NC (numerical control) control section 15, and controlled in such a manner that the spot of a laser beam L follows a machining line K according to a machine processor, while the distance from the machining head 1 to a workpiece W is kept constant. In order to keep constant the distance from the machining head 1 to the workpiece W, the distance sensor 2 mounted on the tip of the machining head 1 measures the distance between the workpiece W and the machining head 1, and feeds back a measuring signal to the NC control section 15, thereby finally controlling the spot of the laser beam L. Connected to a control section 16 is a local operation box 17 which is used at the time of focus position detecting and the like. Numeral 18 indicates a laser oscillator for outputting the laser beam; and numeral 19 a machining apparatus body comprising the three guides for the X, Y and Z axis directions.
Generally, the method used for focus detection is such that a weak laser beam irradiates the workpiece, and a blue flame, derived from a plasma state occurring when the focus of the laser beam is placed on the surface of the workpiece, is visually monitored. In that method, the machining head 1 is allowed to move back by manual operation to widen the distance between the tip of the machining head and the workpiece installed on a machining table so that the tip of the machining head 1 does not interfere with the workpiece during a focus position detecting operation. Thereafter with a focusing program, the machining head is moved relatively and parallel to the workpiece while the laser beam irradiates the workpiece, and an operator finely moves only a lens in the machining head in the plus and minus directions of the Z axis by using the local operation box 17, whereby the blue flame, produced by the plasma occurring near the focus position, is visually monitored.
By using the local operation box of the two-dimensional laser machining apparatus, the operator can input the lens height for which the blue flame is found, as the height of the lens when focused, thereby setting the focus position.
After the focus position has been set, the two-dimensional laser machining apparatus is operated, while keeping the just-focused lens position found by said monitoring and setting operations, and the height of the machining head is adjusted by manual operation so as to provide the proper distance between the machining head and the workpiece.
FIG. 10 is a flowchart showing the steps of a conventional blue flame monitoring operation. In order to perform the operation to monitor the blue flame, after moving back the machining head, the workpiece (or test piece) W is installed on the machining table 14 to determine a focus (S101); the distance between the tip of the machining head and the workpiece W is adjusted so that the tip of the machining head 1 does not interfere with the workpiece or test piece W during the focus position detecting operation (S102); and a program for determining focus is read in the control section 15 (S103). Thereafter, according to the program operation performed by the program for determining focus, the machining head is moved relatively to the workpiece while keeping a constant distance between them (S104); and the laser beam L is irradiated (S105); whereby, while moving the machining head in the Z axis using the local operation box 17 (S106), occurrence of the blue flame is monitored by the operator (S107).
FIG. 11 is a flowchart showing the steps of a conventional focus position setting operation. The blue flame is monitored (S201); whether the blue flame occurs properly is checked (S202); and the operator inputs the height of the current position, when the blue flame properly occurs, as the proper focus height of the machining head, into the control section 16 of the laser machining apparatus (S203). When a proper blue flame does not occur in S202, using the local operation box, the machining head is moved along the Z axis up and down (S204), and the operation beginning with S201 is repeated.
FIG. 12 is a flowchart showing a conventional focusing operation. After the distance between the machining head and the workpiece is widened in S101 shown in FIG. 10, the blue flame occurring is monitored by the operator (S301); whether the blue flame occurs properly is checked (S302); the operator inputs the height of the current position, as the height of the proper focus machining head, into the control section of the laser machining apparatus (S303); and the widened distance between the machining head and the workpiece is adjusted to a proper state (S304). Where a test piece has been used in the above focusing operation, the test piece on the machining table is replaced with the workpiece to start an actual machining operation. Where the above focusing operation has been performed using the workpiece, after completion of the above operation, an actual machining operation is started in that condition.
Conventionally, the distance between the machining head and the workpiece being cut is controlled by a contactless sensor mounted on the tip of the machining head as disclosed in Japanese Laid-Open Patents SHO64-78691 and SHO64-78692.
The former reference discloses a laser machining apparatus which includes a contactless gap sensor, sampling command/drive means and a sampling data storage circuit, wherein, while a machining head is moved away from a reference position from a workpiece in an increment of a specified amount by performing a sampling command with the command means, an output value of the gap sensor is stored in the data storage circuit, whereby the gap sensor output value is controlled in a manner to become the stored data value corresponding to the command gap from a control section.
The latter reference discloses a laser machining apparatus which includes machining head check means, reference output setting means and machining head good/bad judgment means, wherein a machining head is automatically positioned at a check reference position by being given a check command, and a sensor output is measured, whereby the position of the machining head is judged by whether the value is within a predetermined reference output or not. Another device, in which the tip of a machining head is provided with a contactless sensor, and which relates to the control that controls the distance between a workpiece and a machining head during teaching based on a height set in a control panel, is disclosed in Japanese Laid-Open Patent SHO61-165288.
None of the above-mentioned three conventional devices provide continuous monitoring and recording of blue flame, and processing of the data to calculate a focus position.
Although in the blue flame monitoring operation in conventional art a laser beam irradiates a workpiece, and an operator finely moves a machining head in the Z direction using a local operation box such that the workpiece and the machining head are allowed to move relatively to each other at a certain speed, whereby the operator visually monitors the occurrence of the blue flame produced by plasma occurring near the just-focused position, there has been a problem that the monitoring is too complex. Another problem is that the occurrence of the blue flame as detected by the monitoring operation cannot be continuously stored, so that the operator must frequently perform the focus position detection for each monitoring operation to input a current Z axis value through a control section.
Still another problem is that the conventional art focusing is performed by an operator, whereby the detected focus position depends on the degree of skillfulness of the operator, and also whereby the determination of focus position requires non-productive time not accompanied by actual machining. Another problem is that a proper focus position is difficult to determine, whereby a proper focusing cannot be performed, thereby providing a poor machining.